In the field of electronic signal processing many problems exist in measuring the amplitude of Gaussian noise. If a threshold voltage a certain number of db above the rms value of Gaussian noise is employed, linearity is normally very difficult to achieve for low level inputs. This is especially true if the system is designed to operate over a very wide range of noise input voltages. Of the types of systems previously employed the normal peak storage circuits become very non-linear when the input voltage is below the normal breakdown voltage, i.e., forward bias voltage, of currently available diodes. This means that for any input below approximately 200 millivolts the input noise cannot be accurately measured by rectification, especially where very wide band measurement is required. Such requirement exists in laser or short pulse laser receivers.
The present invention approaches Gaussian noise amplitude measurement in a different manner. Instead of trying to measure the high values of noise voltage of the randomly changing noise signal, a servo circuit is employed wherein the input signal which includes noise is supplied to one terminal of a comparator. The output of the comparator is supplied to a limiter, causing the limiter's output voltage level to be +1 when the system input signal is greater than a variable threshold level coupled to the other terminal of the comparator, and to be -1 when it is less than the variable threshold level. If the limiter's output were filtered and the variable threshold were always equal to or greater than zero the voltage value of the limiter's output would correspond to the percentage of time that the system input were greater than the threshold level. A minimum of -1 volt output corresponds to the situation wherein the absolute value of the system input is never greater than the threshold level, and zero volts output corresponds to the situation where the absolute value of the system input is greater than the threshold level 100% of the time. Absolute value may be used since Gaussian noise can be validly considered as being .+-.voltage-time symetrical about zero volts.